winner.js

const WINNING_INDICES = [
  [0, 1, 2],
  [0, 3, 6],
  [0, 4, 8],
  [1, 4, 7],
  [2, 5, 8],
  [2, 4, 6],
  [3, 4, 5],
  [6, 7, 8],
];

/**
  * Calculate if there is a winner in a Tic-Tac-Toe game.
  * @param {string[]} board - Contains either "X", "O" or "" to represent 9 spots on the board
  * @param {string} - Either "X" or "O".
  * @returns {boolean} Whether the player is the winner.
  */
export default (board, letter) =>
  WINNING_INDICES.some((winningIndices) =>
    winningIndices.every((winningIndex) => board[winningIndex] === letter)
  );

winner.test.js

import service from "./service";

describe("game service", () => {
  it("should correctly determine the winner", () => {
    const board = ["X", "X", "X", "O", "X", "O", "O", "X", "O"];

    expect(service(board, "O")).toBe(false);
    expect(service(board, "X")).toBe(true);
  });

  it("should not report a winner if there is none", () => {
    const board = ["X", "O", "", "", "", "", "O", "X", ""];

    expect(service(board, "O")).toBe(false);
    expect(service(board, "X")).toBe(false);
  });
});